In various situations, there are demands for rapid, accurate, and convenient (that is, non-invasive) measurement of an internal temperature instead of a surface temperature of an object to be measured (hereinafter referred to as “measurement object”). Measurement of temperatures of biological bodies, including a human body, is a typical example of those demands. However, it is generally difficult to measure the internal temperature (sometimes referred to as deep/core body temperature, etc.) of the internal region of the biological body, that is, the temperature of the biological body in an internal region that is considered to be held substantially at a constant temperature due to blood flow. When the measurement object is a human body, holding a thermometer at a position where heat is not easily lost, such as under the tongue or arm, reading the thermometer after a thermal equilibrium state is attained between the thermometer and the human body, and then adopting the temperature read as the body temperature, is the common way often used. However, it takes a long time of about 5 minutes to 10 minutes to attain the thermal equilibrium state, and the obtained body temperature does not always match with the internal temperature of the human body. Therefore, it may be difficult to apply such a method to an object having difficulty in undergoing long-term body temperature measurement, such as babies and patients with a certain injury or disease. Further, it is difficult to obtain a body temperature with accuracy high enough for precise body temperature management.
In view of the above, as a thermometer for rapidly and accurately measuring the internal temperature of the human body, there has been proposed a thermometer configured to use a set of sensors including a first temperature sensor that is brought into contact with the body surface and a second temperature sensor arranged so as to oppose the first temperature sensor across a heat insulating member, to thereby obtain the internal temperature based on the temperature measurement results of the respective temperature sensors.
For example, in JP 3863192 B, there is described a high speed accurate temperature measuring device configured to use one set of sensors to mathematically solve a heat conduction equation in a non-steady state, to thereby estimate the internal temperature of the measurement object.
Further, in JP 4600170 B, there is disclosed a thermometer configured to use two sets of sensors, in which a heat insulating member is further arranged between the second temperature sensor (intermediate sensor) and outside air. In this manner, values of the heat fluxes passing through respective sensor sets are caused to differ. In the thermometer of the same patent literature, the internal temperature of the measurement object is calculated based on the output of each sensor in a steady state.